In recent years, reciprocating fluid machines of this type have become smaller in size and weight. Especially in the case of fluid machines applied to refrigeration cycles using CO2 as their refrigerant, the discharge capacity may be small, compared with conventional fluid machines using a fluorocarbon-based refrigerant, and therefore, pistons used in such machines can be significantly reduced in their length along the reciprocating direction. If the size and weight of the fluid machine are reduced, however, side force applied to the side or lateral surface of the piston increases with decrease in the piston length. As a result, the lateral surface of the piston or the inner surface of the cylinder bore wears quickly, lowering the durability of the fluid machine.
To overcome the drawback, a technique has been proposed wherein a Teflon (registered trademark)-based coating layer, for example, is formed over the lateral surface of the piston, an annular ring groove is cut in the lateral surface of the piston coated with the coating layer, and a Teflon-based piston ring is fitted in the ring groove, thereby enhancing the slidability of the piston within the cylinder bore (cf. Unexamined Japanese Patent Publication No. H10-169557).
However, where the piston thus coated with the coating layer is applied to a refrigeration cycle using CO2 as the refrigerant, a problem arises in that the coating layer peels off. This is because the working pressure of the CO2 refrigerant is approximately seven to ten times higher than that of a fluorocarbon-based refrigerant, for example, R134a refrigerant, with the result that the coating layer is drawn with an increased force in the reciprocating direction of the piston.
Particularly, in the case of the piston disclosed in the above publication, the ring groove to be fitted with the piston ring is formed in the grooving step following the coating step of forming the coating layer and the grinding step of uniformly grinding the coating layer. Thus, the ring groove is formed by cutting the coating layer apart, so that the coating layer is liable to peel off from its cut edges while the piston is reciprocating. If the coating layer easily peels off, the lateral surface of the piston or the inner surface of the cylinder bore wears after all, as in the case where the piston has no coating layer formed thereon, resulting in lowering in the durability of the piston, and thus of the fluid machine.
As a conceivable measure, the coating layer may be formed inside the ring groove as well. To coat the ring groove with the coating layer, however, the coating step needs to be again performed following the grooving step. Further, in order to ensure the required fitting accuracy of the piston ring, it is necessary that the coating layer formed inside the ring groove should be ground. In this case, therefore, the piston manufacturing process must be drastically modified, requiring alteration of the piston production facilities and thus entailing increase in cost.
It is also conceivable that a coating material with high coating strength may be used for the coating layer so that the coating layer may not easily peel off. Such a coating material is, however, expensive, eventually leading to increase in the cost of the fluid machine.